The objective of this proposal is to identify the molecular mechanisms that regulate CIITA activity and MHC class II expression. Correct expression of MHC class II on the cell surface is critical for proper antigen presentation and the subsequent initiation of an immune response. Loss of class II expression due to a deletion in the class II transcriptional transactivator, CIITA, results in an immunodeficient disease called Bare Lymphocyte Syndrome (BLS). The mode of action of CIITA is not clear, although subcellular localization and interaction with other proteins may be critical in the mediation of CIITA activity. The specific aims of this grant are to: 1) identify the mechanism by which the mutation in CIITA found in BLS patients results in the loss of CIITA activity; 2) identify the mechanisms which determine the subcellular localization of CIITA; and 3) identify the factors interacting with CIITA and the domains of CIITA which control this interaction. This will be accomplished by analysis of wild-type and mutant CIITA localization in normal and transfected cell lines, characterization of CIITA mutants for their ability to properly localize and modulate class II expression, and examination of the factors associating with and co-purifying with CIITA by subcellular fractionation and immunoprecipitation experiments. Elucidation of the cellular and molecular mechanisms which regulate class II expression will be invaluable for understanding BLS and tumor cell proliferation, and for designing strategies for the treatment of cancer, BLS and other immunodeficient diseases.